Commercial aircraft (e.g., regional jets) are usually equipped with an Auxiliary Power Unit (APU) that provides shaft and pneumatic power to different aircraft systems, including electronics and on-board electrical equipment, avionics, main engine air starters, air conditioning units and the like, before and during the flight. The conventional APU is a small gas turbine engine often located in the aircraft tail, fuselage wing root or fuselage wheel well area. Historically, the first commercial aircraft to have an APU installed was an early (circa 1963) model Boeing 727. At that time there were no ground support equipment at smaller airport facilities to support aircraft operations. Since then, different APUs have been installed in different types of aircraft that operated in smaller and less sophisticated airports. Since the APU allowed aircraft to operate independently from ground support equipment, the APU has contributed to air traffic expansion. The APU has thus been consolidated in civil and also military aircraft. Its function in the aircraft through many decades has progressed from main engine start on the ground to providing a source of electrical and pneumatic power while the aircraft is on ground or during flight.
Due to the increase of APU importance for aircraft operations, it has continually been subjected to improvement and sophistication of its components to achieve improved performance with reduced operational costs, within safe conditions. In order to reach this goal, the integration of the APU in the aircraft is one important aspect that must be considered by both aircraft manufacturer and APU supplier. This integration of the APU in the aircraft constitutes an important and relatively difficult challenge related to an attempt to find an optimal design through a very large number of design constraints.
Every APU has technical specifications that must be observed and complied with during its installation in the aircraft so that the APU can provide its minimum performance during any operational condition. One of these requirements is strongly correlated with the air intake that is necessary for APU functionality. The embodiments disclosed herein are therefore directed toward optimizing air intakes for not only on-board APUs but also air intakes for other related aircraft components, for example an aircraft APU Air Cooling Oil Cooler (ACOC).
A diverter is typically provided with APU air inlet ducts and serves as a shielding device to prevent undesired fluids (e.g., inflammable fluids) from being ingested by the duct. However, the diverter is a fixed structure which thus generates additional aerodynamic drag to the aircraft. In the worldwide context of development of “green technologies” with the aim to reduce pollutant emissions, many research efforts are underway to identify improved solutions to reduce the aircraft aerodynamic drag and therefore lower the fuel consumption.
On prior example to address this problem is disclosed in US Published Patent Application No. 2007/07106479 (expressly incorporated hereinto by reference) which proposes instead of a fixed structure to use a moveable door that is opened only when the APU is operating. Although this prior proposal appears to provide for reduced fuel consumption and noise, there are many technical problems that are associated with such proposal. First, it is a relatively high cost solution due to the mechanical actuators needed to open and close the door. Second, the solution is not simple, due to the electrical or pneumatic system required to install and power the actuators. Third, there is a relatively high impact on aircraft weight as compared to a fixed diverter already existing and installed on many commercial aircraft. Finally, and perhaps most importantly, due to the system complexity, there can be aircraft dispatch reliability issues in the event of a component failure associated with the moveable door.